The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing a substrate.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
One problem encountered in CMP is that a central portion of the substrate is often underpolished. This problem, which may be termed the xe2x80x9ccenter slow effectxe2x80x9d, may occur even if pressure is uniformly applied to the backside of the substrate.
Another problem is the difficulty in removing the substrate from the polishing pad surface once polishing has been completed. As mentioned, a layer of slurry is supplied to the surface of the polishing pad. When the substrate is placed in contact with the polishing pad, the surface tension of the slurry generates an adhesive force which binds the substrate to the polishing pad. The adhesive force may make it difficult to remove the substrate from the pad.
Typically, the substrate is vacuum-chucked to the underside of the carrier head, and the carrier head is used to remove the substrate from the polishing pad. When the carrier head is retracted from the polishing pad, the substrate is lifted off the pad. However, if the surface tension holding the substrate on the polishing pad is greater than the vacuum-chucking force holding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. This may cause the substrate to fracture or chip. In addition, failure to remove the substrate can cause a machine fault requiring manual intervention. This requires shutting down the polishing apparatus, decreasing throughput. To achieve reliable operation from the polishing apparatus, the substrate removal process should be essentially flawless.
Several techniques have been employed to reduce the surface tension between the substrate to the polishing pad. Once such technique is to slide the substrate horizontally off the polishing pad to break the surface tension before vertically retracting the carrier head. This technique may, however, scratch or otherwise damage the substrate as it may detach from the carrier head as it slides off the edge of the polishing pad. The mechanical configuration of the CMP apparatus may also prohibit use of this technique.
Another technique is to treat the surface of the polishing pad to reduce the surface tension. However, this technique is not always successful, and such treatment of the pad surface may adversely affect the finish and flatness of the substrate and reduce the polishing rate.
Another technique is to apply a downward pressure to the edge of the substrate to create a seal that prevents ambient atmosphere from interfering with the vacuum-chucking process. However, this technique may require complex pneumatic controls for the carrier head. In addition, the structure of the carrier head may prevent the application of pressure to the edge of the substrate.
In one aspect, the invention is directed to a carrier head for chemical mechanical polishing of a substrate. The carrier head has a base and a flexible membrane extending beneath the base to define a pressurizable chamber. A lower surface of the flexible membrane provides a mounting surface for applying a load to a substrate. The flexible membrane includes an inner portion and a lip portion surrounding the inner portion, the lip portion positioned and arranged such that, when a substrate is positioned against the mounting surface and the chamber is evacuated to pull the inner portion of the flexible membrane away from the substrate, the lip portion will be pulled against the substrate to form a seal therebetween.
Implementations of the invention may include one or more of the following. The flexible membrane may include a juncture formed between the lip portion and the inner portion. The juncture may be twice as thick as the inner portion. The inner portion may be about 29 and 33 mils thick and the juncture may be about 60 and 66 mils thick. The lip portion may be thicker adjacent the juncture than at an outer rim portion thereof, and may taper from a thickness about equal to the thickness of the juncture to a thickness about equal to the thickness of the inner portion. An edge portion of the flexible membrane may connect the inner portion and lip portion to the base. At least part of the edge portion might fold over the lip portion, or the edge portion might not extend over the lip portion. The lip portion may contact a perimeter portion of the substrate. A retaining ring may surround the mounting surface to maintain the substrate beneath the carrier head. The flexible membrane may be connected to a support structure, and the support structure may be movably connected to the base. An edge portion of the flexible membrane may extend between an outer surface of the support structure and an inner surface of a retaining ring. An edge portion of the flexible membrane may extend around an outer surface of the support structure and across a portion of a top surface of the support structure. The support structure may include a support plate and a clamp, and the flexible membrane may be clamped between the support plate and the clamp. A projection may extend downwardly from a lower surface of the support structure. The projection may be formed integrally with the support structure, or it may comprise a layer of compressible material disposed on the lower surface of the support structure. The lip portion may project downwardly from the flexible membrane to extend past the projection from the support structure.
In another aspect, the invention is directed to a method of chemical mechanical polishing. A substrate is positioned on a mounting surface of a carrier head that includes a base and a flexible membrane extending beneath the base to define a pressurizable chamber, a lower surface of the flexible membrane providing the mounting surface. The chamber is pressurized to urge the substrate into contact with a moving polishing surface, and the chamber is evacuated to pull an inner portion of the flexible membrane away from the substrate and pull a lip portion of the membrane against the substrate to form a seal therebetween.
Implementation of the invention may include pressurizing the chamber to force the inner portion of the flexible membrane outwardly and urge the lip portion of the flexible membrane away from the substrate to break the seal.
Advantages of the invention may include the following. The substrate can be reliably removed from the polishing pad. Underpolishing of the center of the substrate is reduced, and the resulting flatness of the substrate is improved.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.